The purpose of our invention is to deliver an impact actuated warhead to an underwater target, such as a fish, at greater ranges than has been previously possible. As such, prior art exists in two major areas. These are the means for the underwater projection of a warhead to some distance from the launch point, and the means for actuating the warhead upon encountering the target.
In the first area, the prior art consists of devices which expend a major part of their propulsive energy while the warhead and its carrying projectile are in contact with the launching device. Typical examples are spear guns propelling a projectile by means of stored energy in an elastic band, compressed gas, chemical reaction, or springs. Range for these prior art devices depends on projectile mass, drag, and launch velocity. It can be shown by those familiar with differential equations that the velocity (V) of a projectile is given by ##EQU1## where Vo is the initial launch velocity (Fps), t is time (sec), .rho. is the density of water ##EQU2## C.sub.D is the projectile drag coefficient, A is the projectile reference area (Ft.sup.2) and m is the projectile mass ##EQU3## By assuming some typical projectile characteristics, we can examine typical ranges for prior art devices. These are summarized as follows:
Drag coefficient C.sub.D =0.25 PA1 Initial velocity Vo=50 Fps. PA1 Reference area A=0.00545 Ft..sup.2 ##EQU4## Impact velocity for warhead firing=15 Fps. Projectile mass m=0.0155 PA1 1. The launcher and projectile can be made small in size with low cost materials, and no machining operations. PA1 2. There is negligible recoil and no range limitation other than that imposed by the total impulse provided by a particular rocket motor. PA1 3. There are a low number of moving parts (exclusive of springs, there are two moving parts in our launcher and one in our projectile). PA1 4. The projectile is disposable. PA1 5. Reload time is very rapid.
Solving Equation (1) for time (t) where V=15 Fps (the impact velocity) yields a flight time of 0.53 sec. The average velocity over this time is about 30 Fps yielding and approximate range of about 15 Ft. The argument can be made to increase Vo (initial velocity) to increase range but with increasing launch velocity, recoil forces become very large. For an 18" projectile weighing 1/2 pound as in the above example, the acceleration required to achieve a 50 Fps launch velocity is 833 Ft/sec.sup.2. This yields a minimum average launch recoil force of 13 pounds with a peak force of about twice this value. Increasing launch velocity to 100 Fps yields a peak recoil force of over 100 pounds with a range increase to about 37 Ft. Thus, prior art devices such as spear guns are limited in range because of large recoil forces. There is a prior art in an underwater gun with minimal recoil, issued on June 13, 1967 as U.S. Pat. No. 3,324,767. That particular invention reduces recoil by reacting recoil forces on a rearwardly sliding mass. The disadvantages of that invention are the requirement for a relatively thick barrel to contain high pressure gas, the sliding parts which can become fouled, the use of a separate cartridge and projectile, and reload time.
There is a prior art in an underwater gun issued under U.S. Pat. No. 3,580,172 on May 25, 1971. That invention is limited in range to the length of the thrust pole employed.
Our invention relies on the prior art of rocket propulsion in general. It is well known to those skilled in the art of rocket propulsion that recoil forces are negligible during projectile launch when the rocket motor exhaust gases are not contained in the launch device. Also great range can be achieved because the propulsive energy is expended along the flight path of the rocket rather than all at once in the launcher as in the case with prior art "gun" type devices. For the particular application of rocket propulsion to project an underwater projectile at a fish, no prior art was found.
For a given amount of propulsive impulse and a given configuration, range R is inversely proportional to the product of the average drag force F.sub.D and flight time t.sub.F as given by EQU R .about.[1/(F.sub.D .times.t.sub.F)]
The average drage force F.sub.D is proportional to the square of the average velocity V, and flight time t.sub.F is inversely proportional to V. This yields EQU R .about.(1/V)
which states that range is inversely proportional to the average flight velocity. The lower the average flight velocity, the greater the range.
For maximum range, the average velocity must be kept as low as possible. Missiles which expend all their propulsive impulse in the launcher have high initial velocities which decay to some final velocity at their maximum effective ranges. Missiles powered by a rocket motor which do not expend all propulsive impulse in the launcher have lower initial velocities which can either decay or rise to final velocity at their maximum effective range. These rocket propelled missiles have lower average velocities and therefore greater range than those which expend their energy within the launcher.
The advantages over the prior art on using rocket propulsion to propel a projectile to an underwater target can be summarized as follows:
In the second area of prior art concerning the impact actuation of the warhead, there are several inventions for discussion. In U.S. Pat. No. 3,871,120 issued Mar. 18, 1975 entitled "Gun Barrel and Firing Mechanism for Impact-Actuated Underwater Guns" there is described an invention for the purpose of killing large fish, such as sharks. The essence of that invention consists of a detonating plunger attached to a thrust member such as a pole or spear shaft. The detonating plunger slides within a sleeve to which is attached a gun barrel containing a cartridge. To the detonating plunger, a firing pin is attached which is held separate from the cartridge primer by means of a helical spring. In operation, thrusting against a target such as a large fish causes the detonating plunger to slide within the sleeve, compressing the helical spring, and allowing the firing pin to contact the cartridge primer thereby discharging the cartridge and propelling a projectile.
A disadvantage of that invention is that all the internal sliding parts are exposed to water which requires the use of corrosion resistant materials or coatings for the exposed parts. The components of that invention are also costly owing to the number of machined surfaces. Another disadvantage is that when a powered launch technique such as compressed air, elastic, or an explosion is employed, the launch acceleration force tends to cause the plunger to slide within the sleeve and contact the cartridge primer unless the helical spring is made strong enough to resist these forces. However, this is the same spring that must be compressed upon impact to cause this cartridge to fire. Thus, high acceleration powered launch requires higher impact velocity for that invention to operate. A further disadvantage is that a safety pin must be physically pulled to allow the invention to operate. Once pulled, the invention is "armed" even though it has not been launched. Similar disadvantages exist with the impact actuated firing mechanism issued in U.S. Pat. No. 3,300,888, Jan. 31, 1967 titled "Underwater Gun".
The nearest applicable prior art to the rocket propelled projectile and impact actuated firing mechanism of our invention is described in U.S. Pat. No. 3,580,172, Jan. 25, 1971 titled "Underwater Projectile for Firing a Cartridge Upon Impact". The prior art mentioned the use of rocket propulsion on line 24 of column 2 and described a projectile for firing a cartridge upon impact. The projectile consisted of coaxial head and body parts, the head part defining a gun bore having an open aft end for insertion of an ammunition cartridge, the body part having a sliding firing pin which contacts the cartridge primer upon projectile impact with a target. In the preferred embodiment, the slide was free to move in the body without restraint. A modification to the projectile is also described whereas a pliable material which has a central aperture of smaller diameter than the firing pin diameter is used to physically prevent the firing pin from contacting the cartridge primer with sufficient force for cartridge detonation should the projectile be accidentally tipped.
A deficiency in the prior embodiments so described is that a propulsive force imparted to the projectile will cause the slide to position itself at the extreme end of its travel away from the cartridge. As soon as the propulsive force diminishes to a level which is less than projectile drag force, the slide will move forward due to its own inertia against the primer or the pliable blocking material. In this position, a very large momentum change is necessary to fire the cartridge. The firing pin should have a standoff distance from the cartridge primer because compliant flesh "gives" between one and two inches at impact. This is a non-obvious embodiment contained in our invention which is accomplished by a tension spring which holds the slide away from the cartridge during projectile deceleration prior to impact.
Another deficiency with the prior art is that the firing pin slide as described will act as a piston on impact, compressing the air between the cartridge base and firing pin slide which impedes slide motion. Our invention contains a ventilated slide with air passage holes in the slide thereby equilibrating pressure on both ends of the slide. An argument can be made that the inertial slide diameter can be made smaller than the internal bore of the body allowing air passage around the slide. However, our experiments have shown that when a rotating projectile is used such as in our invention, the slightest mass offset from the centerline of the projectile produces inflight accuracy problems.
Yet another deficiency with that prior art is the possibility of accidental discharge in the event the projectile is dropped rather than tipped. Dropping the projectile produces similar momentum changes as underwater impact with a target and a serious safety issue emerges with that prior art. As will be seen hereinafter, our invention contains many advantages over these prior art devices.